Projection-screen.



P. L. CLARK.

PROJECTION SCREEN.

APPLICATION FILED JUNE 26. 913,

1,279,262. Patented Sept. 17,1918.

4 SHEETS-SHEET I.

FIG l2 FIGJ3 WITNESSES WW m 42 7%M5134 I PMQRW P, L, CLARK.

PROJECTION SGREEN.

APPLICATION FILED JUNE. 26. 1913.

1,279,262,. Patented Sept. 17,1918

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g Fame ,5 35 5 5 E 33 5 B 6 5 L 1 L"F'IG.22 F5923 WITNESSES: l/V VEA/ 70/? WMM P. L. CLARK.

PROJECTION SCREEN.

. APFUCATION FILED JUNE 26. 1913. 1 ,279,262. Patented Sept. 17, 1918.

4 SHEETS-SHEET 3.

59 W! TN E 885 8 P. L CLARK.

PROJECTION SCREEN.

-APPLICAT|0N FILED JUNE 261913. 1 79,262. Patented Sept. 17, 1918.

4 SHEETS-SHEET '4.

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WITNESSES: I INVENTOI? UNITED STATES PATE T RAUL-LL CLARK, 0F BROOKLYN, NEW YORK.

OFFICE.

raomcrzomscxwnn.

To all whom it may concern:

Be it known that I, PAUL L. citizen of the United States, residin at Brooklyn, in the county of Kings and tate of New York, have invented new and useful Improvements in Projection-Screens, of which the followin is a specification.

This invention re ates to projection screens for magic lanterns, moving picture machines or other projection apparatus, and especially to that type of screen employing a specular surface provided with a great plurality of accurately curved protuberances closely adj acent'to each other.'

One object of the invention is to effect a very precise distribution of the rays reflected fronrthe screen, so that the image on the screen can be viewed byspectators positioned within a predetermined area or areas;

and in this connection it may be stated that generally the rays from the screen should'be so reflected as to embrace a solid viewing angle which intersects a surface (generally plane), one or more" sides of Whose periph ery is of predetermined position and con figuration.

Otherobjects and special features of the invention are shown in the accompanying specification and drawings and further pointed out in the claims.

In the drawings: Figure 1 is a perspective view of a portion of the screen structure and shows a plurality of curved specular elements made either of glass, silvered on the back, or polished metal, arranged obliquely and in horizontalrows; Figs. 2, 3 and 4 are respectively a front, side and bottom view of an ellipsoid, showing the portion of the ellipsoidal surface employed for the elements shown in Fig. 1; Fig. 5 is a front elevation and Figs. 6 and 7 are sections of Fig. 5,-taken respectively at 15-13 and DD, and show a modified form of element; Figs. 8 and 9 show respectively a. horizontal and vertical crosssection through a curved screen, and illustrate also the proper angles of inclination of the rays reflected from any given point or element on the screen when the rays are required to intersect a given horizontal straight line; Fig. 10 is a geometrical diagram and is used to demonstrate a method CLARK, a

elements on the screen; Fig.

Specification of Letters Patent. P t t t, 17", 1913, Application filed June 26, 19 13. Serial No. 775,992.

the rays downwardly reflected by the saidscreen; Fig. 12 is a front elevation and Fig.

,13 1s a section at F-F, Fig. 12, of a modified form of elements; Fig. ltis a front view of a unit curved specular surface, and 16 are respectively a side and top view of Fig. 14, and show certain details of. construction of a standard element; Fig. 17 is av front view of a unit curved specular surface, and Figs. 18 and 19 are cross-sectional views of Fig. 17, taken respectively at GG andgraphical construction H-H, and show the employed in determining the shape of the specular surface for'a screen deslgned for use in a general case; Fig. 20 is a front elevation and Fig. 21 is a section at KK, Fig. 20, and show modified types of elements; Fig. 22 is a frorit elevation and F ig.'23 is a section of Fig. 22 at L -L,'and show a modified type of element; Fig. 24 is a front view of 'a modified type of element for use in certain cases; Fig. 25 is a front view of a modified t pe of screen for use on an inclined sur ace; Fig. 26 is a front elevation, and Figs. 27 and 28 are sections of Fig. 26 taken respectively at MM and N -N and show a form of element made of transparent material; Fig. 29 is a horizontal cross-section through a curved screen, and shows the approximate paths of incident and reflected rays projected upon curved specular elements on the screen surface, and Fig. 30 is a cross-sectional view at RR, Fig. 29, andshows also the intersection. of the rays with a given surface; Fig. 31 is a front view, and Figs. 32 and 33 are cross-sectional views of Fig.-31,taken P-P, and show the reflection of light rays from an-element similar to'that shown in Figs. 5, 6 and? Fig. 31 is a cross-sectional view showing a modified form' of reflecting surface adapted to reflect light in two direc tions; 'Fig. 35 is a perspective view of a plant showing a projector, a curved screen and a plane surface intersected by the pencils of light reflected and diffused Joy the .36 is a front view of a modified form of specular element,'and Figs. 37, 38, 39,- 4:0 and 41 are sections of Fig. 36 taken respectively at T-T, U-U, V-V, 'V V and W-W; Fig. 42 is a front view of a modified form and Figs. 15"

a projector on one i respectively at QQ and,

of element, and Fig. 43 is a section at S-S, Fig. 42; Fig. 44 is a cross-sectional view of a portion of a screen plate provided with a receding or depressed joining structure between adjacent rows of elements; Fig. 45 1s a perspective view of a portion of a screen plate provided with openings in the joining structure between adjacent rows of elements; Fig. 46 is a front elevation of a portion of a modified form of screen plate provided with openings. o

In the drawings the same parts'appearing in the different views are similarly designated.

Referring first to Fig. 1, it Wlll be noted that I have shown a plurality of small elements 1 in horizontal rows arranged with identical angular relationship to the general screen surface 27; that each row is tipped forwardly so that the top of the elements in one horizontal row is in advance of the bottom of the next adj acenthorizontal row; that the elements in vertical rows are connected by a curved structure 24 which may be called the joining portion; that this said joining portion 24 is convex in relation to the bottom, and concave in relation to the top of the element; that the top portion of the reflecting surface of the element is more nearly'parallel to the general surface of the screen structure than is the bottom portion on account of the reflecting surface curving inwardly from top to hottom, and consequently the bottom portionforms a eater angle with the general screen sur ace than does the top. -The surfaces, or elements, 1- are convex both vertically and horizontally, and may be of spherical, ellipsoidal or other curvature; they are preferably symmetrical with respect to a vertical plane through their center, and also should generally be of uniform size and shape.

In Figs. 2, 3 and 4, 11 represents a portion of. an ellipsoid; it will be noted that the specularsurface 1 (the same as shown in Fig. 1) lies on the surface of the said ellipsoid and below the horizontal plane 6464 passing through. the center of the ellipsoid. It will be noted that the vertical curvature of the surface 1 is less than the horizontal. The position of the element can be anywhere on the surface of the ellipsoid.

In Figs. 5, 6 and 7 the elements 2 are convex vertically and concave horizontally, and

adjacent roWS are retained at the proper degree of inclination by the transverse joining portion 25. The special advantages of this form of element will be mentioned in connection with the description of Figs. 31, 32 and 33.

In Figs. 8 and 9 a projector 13, located on or near the axis of the screen 12, projects rays of light F to fall upon said screen. The mean pencil of rays D, indicated by 9.

single line, strikes an element 1 (considered as a point on account of its relative size to the entire screen surface) in the center of the screen; a portion of these rays is diffused in the pencil R which represents only half the horizontal angle of diffusion, and which comprises only that part of the rays reflected from a narrow transverse strip occupying the center of the element. It is desired that the reflected rays included in the pencil R intersect a strai ht line 17-47 which lies in and preferzfbly bisects a horizontal plane l6only half of which is shown-which has parallel sides 65. The line 1717 I designate the mean viewing line and the plane 16 the mean viewing plane. The mean reflected ray a intersects the line-17=17 per endicularly at the point a the ray 6, re ected slightly sidewise, intersects the line 1717 at b and it will be noted, by reference to the development shown in Fi 9, at a smaller angle of inclination to t e horizontal than the ray a similarly, as the .sidewise or transverse angle of reflection becomes greater and greater, as shown for the rays 1), 0, d, e, f and 9, their inclination to the horizontalprovided they intersect the line 17-17- diminishes. This feature will be further ex plained in; connection with Fig. 10. In reference'to Fig. 9 it should be noted that a, which has been referred to as a reflected ray, may be considered as an end view of an oblique plane which, in reality, is the locus of all the oblique rays a, b, 0, d, etc., emanating from the mean transverse dif ferential stri of the element 1. Furthermore, this ob 'que plane a is practically the locus of the rays reflected from the mean transverse differential strips of all the elements on the screen which are in the same horizontal row as the said element 1. It should be understood that the actual vertical angle of inclination of any ray reflected from the element 1 to pass through any given point in space may be readily il -m:- mined, provided the horizontal angle of reflection of said ray is known. Consider that we know the position on the curved element of a spot which will reflect light to some given point in space, then the an les measured transversely and longitu inally to some other spot on the said element which plane longitudinal to the said surface.

the original angle,

it so as'to pass through a given flected from the'spherical surface plane of the points A B angle measured the point of incidence of the said ray, then 1e-asatwe e degrees,

sothatthe ang e of incidence is nowtwelve plus the original-five degrees. giving a total angle of incidence equal to seventeen degrees; the angle between the incident and reflected ray is now; thirty-flour degrees, 10

which is twenty-four degrees greater than or equal to twice the angle through which the mirror was turned.

ow consider a polished spherical surface (or other surface of revolution) receiving a ray A upon a given point A 'andreflect1ng oint A ,-in space, at a great distance from t e saidsu'rface; then'a second incident ray B, arallel to the first incident ray A, which wi 1 be rethrough a point B inspace, and whic is located upon a line forming an angle of, say, 30 deg. with the reflected ray which passes through A will be that particular ray which is reflected from a point lyingin the aforesaid spherical surface and lying'in the and the center'of curvature of the spherical surface; and the sphere which touch the points of incidence A and B will be 15 deg). The foregoing descrlption may be considered as applying to two points lying in any plane transverse to the curved reflecting. surface, or to two points in Cany onse'quently, any point, D, in space, sag, 16 deg. andlt deg. to the-right o a given A0 1 tan80- (2) OC=OB sec 81,

(4) g;% oos8l.

'From (1), 3% tan from a point D w def. to the right of the points 1 on' the n'xirror' separated to j assra fa,' Figs. 8 and between the radii of the' deg. (one-half of 30- ich is 8 deg. below and? H int on. asphericonvex mirror, wh ch reflects its ray to pass through the, aforesaid given point C. A 15 deg. curved spherical mirror will disperse arallel rays of an incident beam "throng an angle of 30 deg; and any two by an angle of, S m A dog, will reflect their rays 2 deg. apart; and this is generally true regardless of whether the mlrror bespherical, ellipsoidalor of other curvature. v e

In Fig. 10, ZOX, ZOY and XOY represent coordinate planes perpendicular to each other [and intersecting each other at O. The line BC is parallel to CY and corresponds substantially with Fig. 8, while'the int A ma be considered as correspon ing .with the point, or element 1, and thevertical plane ZOY with atangent plane (not shown) to the general screen surface at the point 1. AB represents a reflected'ray corresponding to the 9, and AG represents any ot er ray which is so reflected as to intersect the line BC at C. The angle of inclination 80, of the mean ray is assumed to be own, as are also the values of the following:,ang1es AOB, AOC, AOY, ABC, CBC and B0 are equal. to 90 degs. The side angle of divergence of the ray AC is the angle 81, between the projection of AC and AB on the plane XOY, and the value of this angle 81 is also assumed; the only unknown,

0 7 tan 2:06 and substltutlllg (2), m=

S0; substituting (l) in (4),

(5) tan-82=tan 80 cos 81;

but tan ,80 is aconstant for any particular case, and therefore it can be stated that the tangent of the angle of inclination of anysidewise-difl'used ray, (AC) reflected from the mean transverse differential strip of any element to intersect the mean viewing line (BC), is directly proportional to the cosine of the angle'(81), formed on a horizontal plane between the projection (OB) of the ray (AB) intersecting the mean viewing line (BC) perpendicularly and the projection (DC) of the said sidewise-diffused ray (AG). By the application of the above rule it is possible to determineaccu'rately the'shape of the surface of the element 1, Figs-2, 3 and 4, so that rays reflected therefrom will illuminate an area having parallel sides. This feature will be furtherldescribed in connection with Figs. 17, 18, and 19 which show a graphical construction. In Fig. 10 I have assumed the mean viewing line to be, straight and horizontal and parallel to the general screen surface, and have taken the point C insaid line; it will be be understood, however, that if only" the positionof the said 'point' C in space, and the values of the angles 82 and 81 are known, the value of the angle 80 may be readily determined; consequently, the re- 7 through some other point on the line OX may be found. Thus, if a number of points in a line of any configurationin space are,

known,through which light is to be reflected by a given element, the angle which each spot reflecting light through said line forms with a given spot at some predetermined place on the element may be calculated. On a curved reflectin surface the angle between the two spots wil be one-half the angle between the rays reflected from the said spots.

In Fig. 11, a projector 13 is set up 1n a. building through the window, or opening 21, of which it projects images of any desired character to fall upon the screen 12,- located at a suitable elevation and inclination on the opposite side of the street.

' intersect and embrace an area approximating the shape of the horizontal plane 16, of predetermined outline and assumed to be at the average level of the eyes of spectators on the sidewalk. Similarly, the element P is properly curved, shaped and positioned so as to reflect its rays R to intersect the plane 16 so that approximately all rays reflected from it are directed within the predetermined outline of said plane. It is, therefore, evident that the bounding surfaces of the solid angles of the pencils R and R intersect each other at the mean viewing plane, and a spectator within the'said plane 16 can'see lightrays reflectedfrom both of the elements 1 and P. All elements-on the screen whose use is essential to securing a continuous projected image throughout the plane 16 project their rays in the same manner as the elements P and P, so that the spectator sees the entire image on the screen regardless of his position in the viewing plane. In order to obtain this intersection of the rays from all the elements-the said elements being considered of uniform degree of curvature over the entire screen'surface it is essential that the projector be positioned at a proper distance from the screen, and also that every element on the screen be at exactly the correct angleof obliquity to the rays incident upon it and, therefore, that all portions of the surface of the element be of the requisite curvature and 0bliquity to the incident rays, as will be; fully described in connection with Figs.- 17, 18, 19, 36, 37, etc. Generally the screen may be made spherical or parabolic and all the elements form the same angle with the general surface of the screen at the point where the element is located; but it should also be noted that a flat screen may be employed, made up of elements which become graduall more and more obli ue to the screen sur ace in proportion to t e magnitude of the an le of incidence of the rays projected upon t em. Strictly speaking, the periphcries of intersection of no two pencils of reflected light with the viewing plane 16 will exactly coincide, as will be readily understood by considering the fact that the various elements on the screen are not at uniform distance from any given point in the plane 16; furthermore, the reflected pencils will generally be of variable divergence ow- I ing to the fact that the projector can seldom be located at the exact center of curvature of the general screen surface. In order to partially overcome the slight errors due to the aforesaid failure of the beams to in, tersect each other exactly as desired, I prefer to make the screen so that its curvature can be adjusted slightlythat is, so that itscorners and sides are warped from a true spherical surface, or so that its transverse and longitudinal curvatures are of diflerent rad i. The correct angle of incidence of the general screen surface at any point, with the rays pro ected upon such point may be determined either graphically or analytically provided the angles of transverse and longitudinal diffusion and also the inclination'of the pencil of rays diffused from the specular element at thatpoint are known. When a spherical or parabolic screen is used consid-' era-bl e adjustment may be made by varyin the relative ositions of the projector and screen, to cause a greater or less divergence and inclination of the rays difl'used thereby, in order to attain the most successful distribution for any specific case. The mean viewing plane 16 is of substantially uniform width, preferably, throughout its entire length, the same as the plane 16, Figs. 8 and 9; it should be understood, however, that the surface 16 need not invariably be a plane but may be curved to suit conditions; neither need it be always horizontal, but may be also vertical or oblique. Furthermore, the

ion

periphery of the viewing surface can be I either straight or curved, and the elements can be designed, by following the rules disclosed in this specification, so as to reflect substantially all rays to fall within the said periphery. It should be understood that I do not wish in all cases to confine the rays within a sharply defined polygon, such as a rectangle, rhomboid or triangle, but will consider as being within the scope of the ina matter herein disclosed a straight or curved lines which vention surfaces lying between two or more i may be either parallel or oblique to each 0th r. A considerable degree of accuracy is required not only in shapingthe reflecting-elements on the screen surface but also in assembling the screen itself; but. by'close attention to the personversed in the art, can design and build a screen performing the several functions above enumerated. o

In Figs. 12 and '13 the specular surface of each element 3 is concave both longitudinally and transversely. It will be noted that the portion 26 which joins the elements curves upwardly at the center This will be understood by considering that a ray from a point in the middle of the element will be reflected at a less inclination to the horizontal than a ray from a point atthe side of the said element lying in the same horizontal plane as the middle point. 'This will be clearly understood from the description of Figs. 17,18'and 19.

In Figs. 14, 15 and 16 the unit specular element 1, having a continuously'curved sur:

face, is shown divided into aseries of longitudinal, difl'erentialstrips hkh, etc., and

at the middle of each strip is a differential area 70 etc. which ma be consideredas' themean, elementary area of each longitudinal, difl'erential strip, and which is preferably half-way between the top and bottom of each of. the said strips- These longitudinal strips may be of equal or unequal length and degree of curvature, as desired, although generally the former is preferable. The projection of the vertical angle between the central mean elementary area 70 and themean elementary area in? is the angle In, and the promotion of the horizontal angle is h. As-

suming various values of "the angle. 7b to" satisfy any desired conditions the corre sponding values of .k may be readily deter minded by applyingzthe rules laid down in the description of Fig. 10. It will be noted that the. longitudinal strips M, if, etc, al-

, though of substantially equal length and dc gree of curvatureare in different zones, and consequently each strip reflects the projected lfghtrays incident upon it at a. different inclination; the strip h (considered'as arnit), forexalnple, will reflect the rays incident upon it at a greater inclination to the horizontal than will thestrip kf All of these adjacent strips reflect the rays'to adjacent areas in theviewing surface, resulting in continuous, illuminated area there- 60 upon, so that a person anywhere Within the viewing surface can see reflected light from some $01111, on the said element.

In Figs. 17, 18 and 19 certain rays, a, bf, c, (i etc, comprising a portion of the pencil D the same as shown in Figs. 8 and 9, are projected to fall upon the specular surface cone of rays. Consider all rays in the incithe viewing. plane 16, Figsr8 and 9. The

dimensions 0 an element on the screen are. negligible (it may be considered as a point) in comparison to the distance of the projector fromthe said element; and the angle of the cone of rays leaving the objective and focusing upon a spotthe size of the element and at a great distance from the objective is so small if the aperture of the objective be 21nches iameter and a lantern be 100 feet from the screen the angle of the cone of rays is approximately 0 -3), that the incident rays, for all practical purposes, can be considered parallel. Their'deviation from a true parallel causes a slight increasein the angle of dispersion from the element, the said increase being twice the angle of the dent pencil as parallel; then, if thispencil strikes a-curvedmirror of, say, one degree 'of curvature vertically, the pencil of reflected rays will bediverged throu h an. angle of two degrees vertically. Re erring back to Figs. 8 and 9 of the drawings: it is evident that the difference between the angles of inclination to the horizontal of any two reflected rays is twice the vertical, 0r longitudinal,

angle between the differential zones on the elements from which said rays are reflected. Forexample, if the angle between the lines at and 5 Fig. 9, is, say, two degrees, then the longitudinal "angle between the differential zones from which these rays are reflected onthespecular surface-is one degree. Referring again to Figs. 17, 18 and 19: in Fig.

. 18 the reflected ray a is seen to lie in thevertical plane passing through the axis of the incident pencil (represented by V a single 1 line)- D while the rays 6, c, 45, etc., are diffused sidewise at various angles, the magnitude of which is, of course, twice that of the angle between corresponding points of reflection of the sald rays as measured 1n de- 1 grees curvature of =the' specular surface.

;Having determined the actual angles of inclination o-f the reflected rays (1, b, 0, cl, etc, by means of the method shown-in Figs. Sand 9, and determining also either graphically 1 or analytically the longitudinal and transversef'fdegrees of curvature of the reflecting surface 1 to enable rays reflected from it to embrace the width and length of the viewing surface, points c 6 0, etc, are laid 80 out (see Fig. 17 so that the transverse angle (or what may be considered as'the angle between differential iunes or longitudinal strips) in degrees-curvature of the specular surface between points of reflection of any two rays is one-half the transverse angle of divergence of the said reflected rays. Similarly, the longitudinal angles between the transverse differential strips or zones may be determined, and through the points of intersection of corresponding longitudinal and transverse strips (shown as lines in Fig. 17) a curve 63 is drawn. Parallel rays, 11 ,6}, cf, etc, received upon a differential area of the shape of 63 and lying upon the uniformly curved specular surface 1, will be reflected at the desired different inclinations and will intersect a practically straight, narrow and continuous area or line, such as 1717, in a horizontal plane such as 16, Figs. 8 and 9. The line 17-17 lies within and ordinarily will substantially bisect the plane 16; consequently the transverse strip 63, Fig. 17, will substantially bisect the area of the element 1. By further consideration of the characteristics of this element 1 it is seen that: (a) The rays which are reflected from the central longitudinal strip, or lune, will be at the greatest inclination to the horizontal; whereas, the most widely-diverging side rays, namely, those reflected from the edges of the element, will be at less inclination to the horizontal. (b) Adjacent, differential, longitudinal strips of rays will be reflected through substantially equal longitudinal angles, but at different inclinations. (0) The top and bottom peripheries are of the same general curvature as the mean, transverse strip 63, and corresponding points in the said peripheries are equidistant from corresponding points in the continuouslycurved strip 63. (d) Adjacent areas of the strip 63 control the direction of the rays reflected from them so that they intersect adjacent areas at predetermined locations in the mean viewing surface. (6) The areas so intersected are in predetermined alinement with each other. (f) The element reflects rays through a wide angle horizontally and a narrow angle vertically. (g) The rays are so reflected from the mean transverse strip that they lie in oblique, parallel planes a 6 0 etc., as shown in Fig. 19. (h) The.

elements are oblique and reflect the rays received upon them, from the projecting apparatus, obliquely to intersect a given area of substantially predetermined design and outline. In determining graphically the design of these elements for any specific case, the actual lengths of the rays and the angles they form with each other in order to intersect certain points in the viewingsurface, should be found by development, that is, by the principles of descriptive geometry. Although Figs. 17, 18 and 19 show an element whose reflecting surface is convex both vertically and horizontally, it will be underverse diflerentlal strip 32, of each element bisects the surface of, and is equidistant from the top and bottom peripheries of said element.

In Figs. 22 and 23 the curved elements 5 J on the structure 38 are rectangular in shape and contact each other on all sides so that the joining portion in this modification is only a curved line. The transverse differential strip 33 bisects the element and is equidistant from the top and bottom peripheries of said element.

In Fig. 24 the element 6 is shown bounded by an irregular or brokenperiphery. A screen provided with elements of this shape may be advantageously used where it is desired to distribute the rays over a viewing plane the ri ht-hand half of which is nearer the screen t an is the left-hand half, since the 'axis of the beam of light dispersed from the said right-hand half is at a greater inclination to the horizontal than that dispersed from the other half of the element.

In Fig. 25, 15 is a screen provided with a plurality of specularly reflecting plates 66 having upon their surface a great plurality of elements 1, arranged. in transverse and longitudinal rows. The line 55 indicates the slant of the viewing plane, such as an inclined surface, upon which the light rays are to be reflected and difiused- The transverse rows of elements should be arranged parallel to the line 5-5 in order to obtain the desired distribution upon the viewing surface. The pronounced slant of the viewing plane 55 is such as would be encountered upon an inclined street; and the plates are slanted as shown, so that the zone of light shall substantially parallel the space occupied by pedestrians travelingalong the said street.

e In Figs. 26, 27 and 28, the elements 7 are shown molded upon a glass plate 40. These elements are concave in vertical cross-section and either concave or convex..in horizontal cross-section, and the joining portion between adjacent rows of obllque elements is scalloped, as shown in Fig. 26. The back 37 of the plate 40 is silvered and is preferably flat. Light rays received by elements of this type Wlll be refracted'and reflected as shown by the arrowed lines, and the reflected pencil of light will have substanstantially the same characteristics, as regards distribution, as the pencil reflected from the elements 1, Figs. 1', 17, 18, etc. For example, the rays received upon the central section of the element, at M-M, are diffused at a greater inclination than are those re ceived upon the sides, as at.N-N.

In Figs. 29 and 30 the projecting apparatus13 is positioned at such a distance. from the curved spherical screen 12, whose center of curvature is at the point 60, that the mean ray {that is, the ray from the ap roximate center) B B, B, D and E rom each element is reflected to intersect a given point 56 in the mean'viewing line 17. It'w1ll be noted, Fig. 29, that the rays from the elements are diffused sidewise through angles, equal approximately (approxima-tely on. account of the plan view not being a true develo ment) D, D and D, and vertically through an angle equal approximately D It willbe understood that the elements 3 as shown here aregreatly out of proportion to the actual size of the screen, and are shown thus magnified merely for diagrammatic purposes. If the projector 13 be located at such a distance from the screen 12 as to cause the meanrays to intersect at the point 56, then the limiting rays 0, C "C and C, 0*, C, from other corresponding points on. the elements will intersect each other at points -57 and 59 which do not lie within the surface of the plane 16. Inorder screen is perfectly spherical and the pro jector is placed at its exact center. As pre-. viously noted, I prefer that the curvature and position of the screen and also the osition of the projector be susceptible of sight adjustment, which may be made after the screen is set in position, in order to secure the maximum efficiency from the entire plant. The intensity of the rays reflected from different portions of an element need not be uniform in order for t e spectator'to see a picture of uniform brilliancy, for the obvious reason that his eyes recelve corresponding rays-from all the elements; under these conditions, however, different spectators at a uniform distance from the screen will not ordinarily see equally brilliant images. The elements on the screen may, however, be so designed and proportioned that the pencils, of rays which strike the viewing plane at equal distances from the screen shall be of practically equal intensity; moreover,

the elements may the reflected pencils of rays which strike the viewing plane at a great distance shall be 'of-greater intensity than those near the screen. The'viewing plane will ore th'at'the limiting rays -of thevarious pencils of light reflected fromthe several elements shall intersect each other at or near the desired'points on the periphery of the viewing'plane, I move the projector 13 nearer the screen 12, as by such an arrangement it is possible toso control the reflectedv rays as to confine them almostentirely within the desired periphery of the said plane 16. It should 'be noted, in this connection, that spec-' ta'tors whose eyes Meet the level of the plane 16, and anywhere within its jector should, however, be "always at such a distance from the screen as to "cause a convergence of reflected rays from'corresponding points of the elements,

noted in connection with Fig. 11. The intensity of the light diil'used from the ele ments located on different portions of the screen will vary somewhat owing to the fact that all the elements do not receive their rays at the same angle except when the periphery, are enabled to seethe entire pro ected a p10" positioned on the axis of as. previously dinarily be rhom'boidal in shape, but may, .of course, assume any shape desired. The proper shaping of the peripheries of the elements is very important, as only by limbe so proportioned that striking it j iting the areaand outline of the curved specular surface can the confinement of the diffused ras within a surface of predetermined outline be accomplished. Where the screen: is located below the viewing plane, it is evident that the said screen may be turned upside down and'the projector positioned either above or below the said viewing plane.

In Figs. 31, 32 and 33 the oblique eleinents 2 on'the structure 28 areof convex curvature longitudinally .and of concave curvature transversely; adjacent transverse rows areunited by the oblique joining por tion 25. The elements are bounded'on their longitudinal sides by circular arcs 67, substantially parallel .to each other, and on the top and bottom by eccentric arcs of substantially equal radii. Although specifying circular arcs, it will beunderstood that other curves may be used where expedient. The reason the joining edges are curved instead of straight will be understood by analyzing the configuration of the line of intersection of two similar intersecting concave or con- ,the said mirror be cut off this plane.

vex surfaces, such, for example, as two 6- inch parabolic or ellipsoidal mirrors. Consider that a plane parallel to the axis of such a mirror and 2' inches away from its axis intersects the mirror a gi to coincide with mirror in preteen let these two Cut the other clsely the same manner;

mirrors be placed together onthe out surfaces, when it will be observed that the line ven line and that of contact is curved. This line of contact, when occurring alon a vertical plane between two adjacent similar elements on the screen herein described is referred to as a curved joining edge. Substantially paralrections, aswill also a small portion 'of the rays striking near the point P"; and in order to overcome any such undesirable reflection the portion 25 may be blackened or roughened, as it is well known that such treatment will render dull those portions of a reflecting surface which without said treatment would appear bright. The element2 should be designed in accordance .with the principles stated in connection with Figs.

10, 17 18, 19, etc.; and a screen provided with a great plurality of these elements will be extremely efficient, as substantially every ray received upon the surface of the screen is projected by a singlereflection to fall within the area of the viewing plane. The joining portion 25 may be made normal to the general screen surface or it may slant downwardly from the front, as shown in Fig. 44. The function of the joining portion is primarily to serve in holding adjacent horizontal rows of the reflecting ele-- ments in their proper ositions, and to render possible the manu act-ure of a plate or sheet which shall contain a large number of the small elements all rigidly fixed with relation to each other and to the general plane of the plate itself. For ease of manufacture the quite oblique 'oining ortion will be found preferable to t at whic is substantially normal to the general plate surface (on account of rupturing the material in theprocess of stamping or rolling) but on account of double reflection of the light rays (described in connection with Fig. 34) it is generally desirable that the obliquity of the joining ortion do not depart more than a few degrees from a normal to the screen surface, in order that the least loss of light and double reflection shall be caused by its presence. 7

In Fig. 34 a portion a of a pencil of rays is shown incident upon the lower part of the curved element 8 and also upon the curved joining portion 43. Certain of the rays (1.

are once reflected, as shown by the'arrowed lines a; but the rays a, incident upon the joining portion 43 and upon the very bottom ing area and, consequently,

of the curved element, are twice reflected and emerge from the screen surface at a less inclination than the rays 0.". It should be understood that the lines w represent merely the axes of the small, encils of the incident and reflected rays; an it will be appreciated that the reflected pencil of rays a will be diverging to. such an extent that spectators in its path will also be in the path of rays reflected from similar parts of other .elements and their 'oinin portions. The rays a are reflected t roug an angle embracing the mean viewing area or plane, while the rays a are reflected to embrace a secondary viewing area; and their function is to enable the operator of the projecting apparatus (ordinarily located at a considerable elevation above the viewing surface) to note the appearance of the pictures projected upon the screen. The rays projected into the secondary viewing angle go in a positive, and not merely an incidental direction, and through an angle of desired and predetermined divergence; furthermore, although the intensity of the said secondary beam of light should be substantially the same as that of the beam directed into the principal viewing angle, such a restriction is obviously unnecessary. Unless the joining portion 43 is very oblique, say, at an angle of about fortyfive degrees with respect to the general screen surface, the rays influenced by it will ordinarily be reflected outside the principal viewno unevenness of illumination in said. area will occur. The principles of double reflection may be readily applied to the rays dispersed into'the princi al viewing area; but, on account of the a ditional losses caused by double reflection, a decided advantage is gained by the use of elements so curved and proportioned as to accom lish the desired distribution by a single reflection only.

In Fi 35 a projector 13 sends its rays F to all upon the concave screen 14. From two elements, represented by points,

P P, pencils of rays R, R, are reflected to embrace a long narrow viewing plane 18. This plane, although preferably of. substantially uniform width throughout, may be of rhomboidal, triangular or other shape. The control of light rays to embrace the surface 18 should be such that rays from corresponding portions of the elements intersect each other at or near the said surface, the same as described in connection with Fig. 11. In Fig. 11 the viewing plane runs, however, in

. a direction substantially parallel, while in Fig. 35 it runs perpendicular, to the general screen surface.

In Figs. 36, 37, 38, 39, 40 and 41'the element 9 is convex in longitudinal crosssection, as shown in Fig. 37, and concave in transverse cross-section, as shown in Figs. 38, 39, 40 and 41. Small portions,

68, 69, 70, 71, of the entire pencil of rays received upon the element are of equal cross-section (that is, thenumber of rays in, and the intensity of, these said portions 68, 69, and 71 are the same for all). By reference to Fig. 37 it will be noted that the longitudinal Curvature of the element varies, being of sharp curvature at the top where the surface is most nearly parallel to the general screen surface, and gradually flattening out at the bottom where the surface is most oblique. Referring to Figs. 38, 39,40 and 41, it is seen that in transverse cross-section the element is quite shallow at the top and gradually becomes deeper to- .ward the bottom. An element constructed along these lines can be made to produce a most uniform distribution of the rays: for example, the pencil of rays 68 will be reflected through a comparatively large longitudinal angle 72, and a narrow transverse angle 76; the pencil 69 will be reflected through a narrower longitudinal angle 73 and a wider transverse angle 77 than the pencil 68; and so forth; the product of the degrees of longitudinal curvature by the degrees of transverse curvature of all the transverse strips on the elements surface being equal. That is to say, each of the pencils 68, 69, 70, 71, will be diffused through sub-' stantially equal solid angles, although the width and depth of the angles vary. The intensity of a pencil of parallelrays of light reflected from a differential or minute area of a specular surface curved in two or more directions-such as a surface of revolution is inversely proportional to the product of the degrees of curvature longitudinally by the degrees of curvature transversely, pro vided the incident pencil of rays strikes the said minute area normally; for example, if the curvature of the minute area is, say, two degrees each way, the intensity will be equal to and if three degrees by tensity will be equal to 1 1 3 x 4 S 12 When, however, the incident pencil strikes the said minute or differential area obliquely the intensity of the reflect-ed pencil varies also as thecosine of the an le of incidence of the said pencil with the di erential area. In order that the entire beam of light may be reflected from all portions of the surface of a curved specular element with strictly uniform intensity, it is necessary that each differential area of said element be of such curvature, and at such an angle to the rays incident thereon, that the degrees of curvature longitudinally, multiplied by the degrees our degrees, the inof curvature transversely, multiplied by the secant (the reciprocal of the cosine) 0f the angle of incidence of the rays on the said differential portion of the element, be a constant. This principle of constructing elements so as to obtain uniform distribution of the reflected light rays may be applied in determining the shape of the various specular elements herein described. In practice the rule need not necessarily be applied to infinitesimal or (lifferential areas, but to entire strips extending across the surface of the element, so that the product of the average degrees of transverse curvature, of a given strip, by the average degrees longitudinal curvature, by the secant of the ngle of incidence, is substantially the same for all narrow strips on the surface of the element. In surfaces of this character it is evident that the portion of the surface which is'most' nearly normal to the incident rays has the sharpest curvature, whereas,-the portions which receive the projected rays most obliquely are of lesser curvature. Elements of the type above described may be advantageously used on the screen 14, Fig. 35, to diffuse the rays over a long narrow area; and the curvature of the various parts of the element 9 may be modified to give any desired intensity of the reflected rays upon different portions of the viewing surface.

In Figs. 42and 43 the elements 10 are substantially diamond shaped and have a curved joining portion 23 at the bottom. This element is convex longitudinally, as shown in Fig. 43, and may be either concave or convex transversely; when of ellipsoidal shape and curvature it will reflect rays so as to accomplish nearly the same distribution as the element shown in Figs. 36, 37, etc.

In Fig. 44 the joining portion 45 slants downwardly from the front so that all incident rays 46 strike the surface of the elements 44 and are, consequently, reflected exactly as desired. A small percentage of the reflected rays will strike the said portion 45; but they will be reflected away from the desired viewing space.

In Fig. 45 the portion 47, connecting transverse rows of elements 48, is provided with an aperture 49, which permits the escape of undesirable accumulations, such as rain or dust, from the elements surface. A screen provided with the openings 49 also offers less resistance to the wind than one made of a solid, unperforated sheet of glass or metal. As these apertures are in the joining structure, which is substantially normal to the general screen surface, their presence does not result in any loss of light. Screen plates constructed with apertures of any desired character may be madeby the processes used in forming expanded metal sheets.

In Fig. 46 the screen structure 50 consists of a plurality of spherical or ellipsoidal elements 5]., between certain portions of which I provide perforations 52. A screen made of plates of this description will offer less resistance to wind pressures than will one havin no perforations. 6 Alt ough the various views of the elements show their peripheries, where the metal is bent, as sharp lines, it will be understood that, in practice, these edges w1ll ord1- narily be more or less rounded, causing gen- 10 eral diffusion of-asmall percentage of the rays both within and without the limits of the desired viewing area. The lossresulting from the presence of these said curved edges will, however, be inconsidei 'able and does not affect the principle of the specular dispersion over a limited and definite viewing area of all rays reflected from what is to be considered the working or eflicient part of the screen surface.

Instead of using a polished metal structure, the elements may be made of glass, of uniform or variable thickness, and having a reflecting back surface.

Large screens must necessarily be built up of a plurality of plates, and in assembling these 'plates it is essential that the distance between the nearest rows of elements, or corresponding parts of adjacent elements on any two adjacent plates be the same as that between adjacent rows on the body of the same plate; as otherwise streaks will appear whichwill be either dull or brilliant according to whether nearest corresponding parts of elements in adjacent rows are too 85 close together or too far apart. The plates may overlap each other, the same as wallpaper, or may have abutting edges. The plates may be supported upon a suitable backing of wood or metal by screws or nails 40 provided with polished heads of the configuration of the reflecting elements. A framework or shadow-box of necessary depth may be used, where desired, to shield the screen from undesirable sidelights. Befleeting plates provided with the improved form of elements of the proportions and obliquity described herein, may be cut in the shape of sign letters or characters and illuminated from any desired source or from a projector equipped with a slide bearing the design of said characters.

It is possible to apply certain features shown in some modifications generally to other modifications of the elements, and two or more forms of elements arranged alternately or in alternate rows may be used on the same screen to reflect light to several viewing areas; or one style of element may be combined with portions of another form. 00 Where screens of considerable height are used the elements at the top may be of less depth of curvature transversely and greater depth longitudinally than elements at the bottom of the screen, and the transverse rows of elements, from top to bottom of the screen may be of curvature varying gradually between that suitable for the said to and bottom rows. 7

hen desirable that the operator of the projecting apparatus be enabled to note the actual condition of the image on the screen, as viewed from the mean viewing surface, a flat or curved mirror may be positioned obliquely at some suitable place in the path of the rays directed to said surface, so as to reflect? upwardly to the said operator a portion of the rays comprising sald image.

The reflecting portion of the elements should be a generated or warped surface or a surface of revolution, or of such nature that it can be accurately duplicated by machinery, so that all the elements on the' screen, or at least, on a certain area of the screen shall be identical.

In order to design an element which will reflect the rays incident upon it so as to cover a given length and breadth of viewing area certain facts must be known, namely: (a) The elevation above the view-* ing plane of the centre of the screen; (b) the approximate length and breadth of the viewing area; (0) the horizontal and vertical distances of the mean viewing line 17-17, Figs. 8, 9, 29 and 30 from the screen center (the same as the position of the middle element on the screen). The size of the element should be such that when seen from the viewing plane, its outline will be indistinct-so that the surface of the screen will appear smooth and continuous (if viewed from a distance of about 150 ft. the elements should not be larger than one-half inch square, in order to avoid giving a checkered appearance to the projected picture). Assume (a) the middle element of the screen, on the front of a building (see Fig. 11), to be at a height of 50 ft. above the viewing plane; (6) this element to be 125 ft. across the street from the mean viewing line (corresponding to 1717, Fig. 8); (0) the length of the plane to be 280 ft.- the same as the mean viewing line 17-47 which parallels the general surface of the screen-and the width of this area to be 30 ft. The general arrangement of the plant 115 to correspond with that shown in Fi 8 and 9, which show the arrangement r a symmetrical system; or to Figs. 11, 29 or 30, which show practically the same thing. Referring to the description of Figs. 8 and 120 9; the mean viewing line 17- -17 will be (substituting the values above assumed) tan =22 deg.

that the middle ray from the. middle of the middle element which intersects the middle of the viewing 1plane will have a slant of 22 deg. to the orizontal and will be directed so that it lies in a vertical plane.

which is perpendicular to the screen at its middle point. As the incident-ray has been assumed to strike the screen at its middle deg.=11 deg. v I

from the vertical. (If a small plane mirror were placed at the middle of the screen and were to reflect an incident pencil of rays to intersect the middle of the viewing plane, this plane mirror would have to be downwardly tipped 11 deg. from a true vertical position. If a horizontal ray strikes a plane vertical mirror perpendlcularly the ray will be reflected back upon itself; if the mirror be tipped five deg. the ray will be reflected ten deg. from its path of incidence. Instead of'basing the various calculations in this specification on tipping plane mirrors a given angle in an dlrection with respect to a given fixed irection of an incident ray, it will be seen that I have generally considered the points on the curved specu arsurface as being a certain number of degrees of surface curvature away from a fixed or predetermined point, as noted in the description of Figs. 17, 18 and 19. This reflected ray therefore strikes the viewing plane at an angle of 22 deg. The pencil of rays must be wide enough to embrace the width of the viewing plane, and since the element is 135 ft. away and the viewing plane is at an angle of 22 de to the ray incident upon its center, the vertical angle of dispersion caused by an infinitesimal vertical strip extending up the mid:

dle of the element from top to bottom, is

5 deg. (by the trigonometric formula for solvin an oblique triangle, given two sides and t e included angle). As this vertical dispersion is .5 deg., it is evident that the total vertical curvature of the element is 3- deg. =2.5 deg,

or, substantially 1.25 deg. above and 1.25 deg. below the middle point of the element. Proceeding in a like manner, it'is found that in order for the dispersed pencil of rays to cover the entire length (140 ft. each side of the center) of the 280 ft. viewing plane, the rays must be dispersed sidewise through an angle of 92 deg. The only remaining factor is the plotting of a; suflicient number of points to determine the configuration of the mean transverse differential strip, and this is done, one point at a time, (three points each side of the middle point of the element ordinarily suflicing) by substituting known or trigonometrically derived values in the formula,

tan 82=tan 80 cos 81,

as pointed out in the description of Fig. 10 of the drawings.

I know that a projection screen employin curved specular elements, arranged ob ique to the axis of the screen, is not new, as this forms the basis of my application, Serial Number 585,532, but what I claim is 1. A projection screen provided with a surface composed of small curved specular elements arranged in transverse rows, the general surface of each of the said elements being oblique to the general surface of the screen, a joining structure connecting adj acent transverse rows of elements and disposed substantially normal to the general screen surface, the said joining structure being convex at the bottom of each element and concave at the top; substantially as described.

2. A projection screen provided with a great plurality of specular elements each comprising a portion of a surface of revolution, the most oblique portion of said surface being at the lowest point of the element; substantially as described.

3. A proiiection screen provided with a great plura ity of specular elements of convex curvature longitudinally, and concave curvature transversely, adjacent transverse rows of said elements being connected by a scalloped joining structure; substantially as described.

4. A projection screen provided with a plurality of specular elements each comprising a. ortion of a surface of revolution and o lique to the general screen surface, the most oblique portion of the surface of each element being at its lowest point; substantially as described.

In witness whereof, I have hereunto set my hand this 25th day of June, 1913.

PAUL L. CLARK.

Witnesses:

D. F. Monnous, Cass. Gum. 

